The Hamburger Bahnhof was definitely a hidden gem.
Tons of different artwork, and don't even attempt to see everything in one visit. We went late one afternoon (they were open until 8pm on Thursdays) and must have spent at least 3 hours there. Almost got lost at one point, the museum goes back into some old train tunnels.
http://www.smb.museum/en/museums-and...nhof/home.html
The main Berlin station is worth looking at also. Quite impressive.